In a large bowl, cream together butter, white sugar, and brown sugar until smooth. Add eggs, egg yolk, and vanilla and mix until fluffy. Stir in peanut butter. Sift together flour, baking soda, and salt and stir into peanut butter mixture. Refrigerate for at least 2 hours.

Roll dough into walnut sized balls and place onto greased baking sheets. Flatten slightly with a fork dipped into powdered sugar. Bake at 350° for 12-15 minutes. Allow to cool a few minutes before removing to a wire rack to cool. Do not over bake!